Looking back through old posts, I found this.

If the scum doesn't go away with any of those methods, it could be sulphur. Just wipe it off with a soft brush or cloth while still in the water. This happens sometimes with polysulfide toners - and old hypo (as in Hypo-alum toners). Don't bother with the acid, it's not going to help.

Another possibility is Calcium Sulphate (=gypsum). In the amounts that are relevant here, it will readily dissolve in COLD water. It is a lot more difficult to dissolve it in hot water.